1. Advanced Pulmonary Mechanics during Mechanical Ventilation

2. Points of Discussion Basics Abnormalities Equation of motion
Dynamic Compliance
Pressure-volume loop
Flow-volume loop
Work of breathing
Lower and upper inflection points
Hysterexis
Intrathoracic Pressures
Air-leak
Air trapping
Increased airway resistance
Inadequate flow support
Inadequate sensitivity
Atelectasis
Inadequate PEEP
Airway obstruction
Over-distension

3. Tube + Spring Model
Resistive Forces
Elastic Forces

4. Static and Dynamic Pressures
PIP
Flow-Resistive
Pressure difference (Pres)
Pplat
Alveolar Distending (recoil)
Pressure difference (Pdis)
PEEP
time

5. Pressure difference = Flow Rate x Resistance
Airway Resistance
Pressure difference = Flow Rate x Resistance
dP = Q x R
R =
8 L (visc.)
p r 4

6. Resistive and Elastive Forces
DYNAMIC CHARACTERISTICS:
dP = dV / Cdyn
RESISTANCE:
dPresistive = R x Flow
STATIC COMPLIANCE:
dPdistensive = dV / Cst
dP = dPresist. + dP dist.
dP = R x Flow dV / C st

7. Basic Calculations dP = R x Flow + dV / C st Cst = dV / (Pplat-PEEP)
Pressure
Cst = dV / (Pplat-PEEP)
R = (PIP-Pplat) / Flow
Pplat
PEEP
time

8. Assessment of static P-V curve Super-syringe method:
Stepwise inflation from a big syringe with multiply occlusions at each volumes to record recoil pressure
Time consuming
Cumbersome to perform
Difficult to standardize
Patient must be paralysed, connected to a special equipment
Great risk of oxygen desaturation
Volume
Pressure

9. Assessment of static P-V curve Slow Flow Single Inflation Method
Slow (5-10 lpm) inspiratory flow with large Vt and ZEEP
The inspiratory curve of the dynamic P-V loop closely approximates the static curve
The flow-resistive pressure component could be subtracted
Easy to perform, fast and relatively comfortable
Servillo: AJRCCM 1997
Lu: AJRCCM 1999
Volume
Static curve
UPIflex
inspiration
LPIflex
Pressure

10. FRC and PV Loop Normal Compliance TLC FRC FRC VOLUME Negative Positive
DISTENDING PRESSURE
Normal Compliance
FRC
FRC

11. Components of Pressure-Volume LoopVT
Expiration
Volume (mL)
Inspiration
PIP
Paw (cm H2O)

12. Pressure-Volume Loop (Type of Breath)
Vol (ml) I E I I
Paw
(cm H2O)
Controlled
Assisted
Spontaneous
I: Inspiration
E: Expiration

13. PEEP and P-V Loop VT
PIP
Volume (mL)
PEEP
Paw (cm H2O)

14. Inflection Points
Upper Inflection Point: Represents pressure resulting in regional overdistension
Lower Inflection Point: Represents minimal pressure for adequate alveolar recruitment
Upper Inflection Point
Volume (mL)
Lower Inflection Point
Pressure (cm H2O)

15. Decreased Compliance
Normal
Patient
Volume(ml)
Pressure (cm H2O)

16. Lung Compliance Changes and the P-V Loop
Volume Targeted Ventilation
Preset VT
Increased
Normal
Decreased
Volume (mL)
Paw (cm H2O)
PIP levels

17. Lung Compliance Changes and the P-V Loop
VT levels
Increased
Normal
Decreased
Pressure Targeted Ventilation
Volume (mL)
Paw (cm H2O)
Preset PIP

18. Hysteresis Normal Hysteresis Abnormal Hysteresis Volume (ml)
Pressure (cm H2O)

19. Flow-Volume Loop Inspiration PIFR FRC VT PEFR Expiration Volume (ml)
Flow (L/min)
PEFR
Expiration

20. Positive Pressure Ventilation: The Equation of Motion
In a passive subject, airway pressure represents the entire pressure (P) applied across the respiratory system.
The work required to deliver a tidal breath (Wb) = tidal volume (VT) x airway pressure
The pressure (P) associated with the delivery of a tidal breath is defined by the simplified equation of motion of the respiratory system (lungs & chest wall):
P = VT/CR+ VT/Ti x RR + PEEP total
P elastic
P resistive
P elastic
Where CR = compliance of the respiratory system, Ti = inspiratory time and VT/Ti = Flow, RR = resistance of the respiratory system and PEEP total = the alveolar pressure at the end of expiration = external PEEP + auto (or intrinsic) PEEP, if any. Auto PEEP = PEEP total – P extrinsic (PEEP dialed in the ventilator) adds to the inspiratory pressure one needs to generate a tidal breath.

21. Work of Breathing B A: Resistive Work B: Elastic Work A Volume (ml)
Pressure (cm H2O)

22. Work of Breathing
WOB is a major source of caloric expenditure and oxygen consumption
Appr. 70% to overcome elastic forces, 30% flow-resistive work
Patient work is a one of the most sensitive indicator of ventilator dependency
Comparison of Ventilator and Patient work is a useful indicator during weaning process
WOB may be altered by changes in compliance, resistance, patient effort, level of support, PEEP, improper Ti, demand system sensitivity, mode setting
Elevated WOB may contraindicate the weaning process

23. WOB Measurements WOB = ∫0 ti P x Vdt B C E D A Elasic work: ABCA
Resistive work
Inspiratory: ADCA
Expiratory: ACEA B C E D A P

24. Work of Breathing Measurements
WOB = ∫0 ti P x Vdt
Paw: Ventilator Work: The physical force required to move gas into the lung, represents the total work of the resp. system (patient + ventilator)
Peso: Patient Work: done by respiratory muscles, represents the pulmonary work of breathing
Paw-Ptr: Imposed Work by the Endotracheal tube

25. P-V Loop and WOB V P V V P P Normal Compliance Decreased Compliance
Increased Resistance
Decreased Compliance
Normal Resistance P V
Normal Compliance
Normal Resistance V P P

26. Work of Breathing
Work per breath is depicted as a pressure-volume area
Work per breath (Wbreath) = P x tidal volume (VT)
Wmin = wbreath x respiratory rate
WEL = elastic work
Volume
Volume
Volume
WR = resistive work VT
Pressure
Pressure
Pressure
The total work of breathing can be partitioned between an elastic and resistive work. By analogy, the pressure needed to inflate a balloon through a straw varies; one needs to overcome the resistance of the straw and the elasticity of the balloon.

27. Intrinsic PEEP and Work of Breathing
When present, intrinsic PEEP contributes to the work of breaking and can be offset by applying external PEEP.
Volume VT VT
Dynamic
Hyperinflation
FRC
Pressure
PEEPi
PEEPi = intrinsic or auto PEEP; green triangle = tidal elastic work; red loop = flow resistive work; blue rectangle = work expended in offsetting intrinsic PEEP (an expiratory driver) during inflation

28. The Pressure and Work of Breathing can be Entirely Provided by the Ventilator (Passive Patient)+ + ₊ ₊ + +

29. Paw = Airway pressure, Pes= esophageal pressure
The Work of Breathing can be Shared Between the Ventilator and the Patient
The ventilator generates positive pressure within the airway and the patient’s inspiratory muscles generate negative pressure in the pleural space.
AC mode
PAW
patient
machine
PES
time
Paw = Airway pressure, Pes= esophageal pressure

30. Work of breath Paw Pes Pressure Work to inflate the chest wall Volume
Resistive Work
Elastic Work of Lung
Pressure
Elastic Work of Chest
Paw
Pes
Work to inflate the chest wall
Inflation
Deflation
Volume

31. Relationship Between the Set Pressure Support Level and the Patient’s Breathing Effort
The changes in Pes (esophageal pressure) and in the diaphragmatic activity (EMG) associated with the increase in the level of mask pressure (Pmask = pressure support) indicate transfer of the work of breathing from the patient to the ventilator.
Carrey et al. Chest. 1990;97:150.

32. Partitioning of the Workload Between the Ventilator and the Patient
How the work of breathing partitions between the patient and the ventilator
depends on:
Mode of ventilation (e.g., in assist control most of the work is usually done by the ventilator)
Patient effort and synchrony with the mode of ventilation
Specific settings of a given mode (e.g., level of pressure in PS and set rate in SIMV)

33. Intrathoracic pressures
PROX. AIRWAY PRESSURE
TRACHEAL PRESSURE
PLEURAL
PRESSURE
ALVEOLAR PRESSURE

34. 3 Levels of Lung Mechanics
DYNAMIC CHARACTERISTICS:
dP = dV / Cdyn
RESISTANCE:
dP = R x Flow
STATIC COMPLIANCE:
dP = dV / Cst
AIRWAY RESISTANCE:
dP = Raw x Flow
LUNG COMPLIANCE:
dP = dV / CL
IMPOSED RESISTANCE:
dP = Rimp x Flow
CHEST WALL COMPLIANCE:
dP = dV / Ccw

35. A closer look at lung mechanics
Crs = Vt / dPdist (aw)
Ccw = Vt / dPdist (pl)
CL = Vt / Pdist (aw - pl)
or (tr - pl)
Rrs = Presist (aw) /Flow
RL = Presist (tr) / Flow
Rimp = Presist (aw-tr)/ Flow
Paw
Ptr
Ppl P
dP resist
dPdist t

36. Respiratory Mechanics in ARF*
Volume
Reduced range of volume excursion: Low compliance
Flattering at low and high volumes: Lower and upper inflection points
*Bigatello: Br J Anaest 1996
NORMAL
ARDS
Pressure

37. Lung Protective Strategy
Set PEEP above the lower Pflex to keep the lung open and avoid alveolar collapse
Apply small Vt to minimize stretching forces
Set Pplat below the upper Pflex to avoid regional overdistension
Volume
Pressure

38. Abnormalities Air-leak Air trapping Increased airway resistance
Inadequate flow support
Inadequate sensitivity
Atelectasis
Inadequate PEEP
Airway obstruction
Over-distension

39. Air Leak
Volume (ml)
Air Leak
Pressure (cm H2O)

40. Air Leak Inspiration Expiration Air Leak in mL Normal Abnormal
Flow
(L/min)
Volume (ml)
Air Leak in mL
Normal
Abnormal
Expiration

41. Air Leak
Volume (mL)
Time (sec)

42. Air Trapping Inspiration Normal Patient Time (sec) Flow (L/min) }
Auto-PEEP
Expiration

43. Air Trapping Inspiration Expiration Normal Abnormal Flow (L/min)
Does not return
to baseline
Volume (ml)
Normal
Abnormal
Expiration

44. Response to Bronchodilator
Before
After
Time (sec)
Flow (L/min)
Long TE
PEFR
Shorter TE
Higher PEFR

45. Increased Airway Resistance
Inspiration
Flow
(L/min)
Volume (ml)
Normal
Abnormal
“Scooped out”
pattern
Decreased PEFR
Expiration

46. Increased Raw Normal Slope Lower Slope Higher PTA Pressure (cm H2O)
Vol (mL)
Normal Slope
Lower Slope
Pressure (cm H2O)

47. Inadequate Inspiratory Flow
Volume
(ml)
Active Inspiration
Inappropriate Flow
Paw (cm H2O)

48. Airway Secretions/Water in the Circuit
Inspiration
Flow
(L/min)
Volume (ml)
Normal
Abnormal
Expiration

49. Airway Obstruction F F V V
Before Suction
After Suction

50. Optimising PEEP V V P P
PEEP: 3 cmH2O
PEEP: 8 cmH2O

51. Inadequate Sensitivity
Volume (mL)
Paw (cm H2O)
Increased WOB

52. Atelectasis
Lost FRC
Replaced FRC V V P P

53. With little or no change in VT
Overdistension
With little or no change in VT
Normal
Abnormal
Volume (ml)
Pressure (cm H2O)
Paw rises

54. Overdistension
Overdistension occurs when the volume limit of some components of the lung has been exceeded
Abrupt decrease in compliance at the termination of inspiration
Results in a terminal “Beaking” of the P/V Loop
Volume
Pressure

55. Overdistension Index
Volume
C20
Cdyn
Pressure
0.8 Pmax Pmax

56. Thank You